Chinese internet users are rebelling against an internet crackdown brought in on the anniversary of the Tiananmen Square massacre.

Twenty years after the pro-democracy protests that claimed the lives of hundreds – or even thousands – of unarmed civilians in Beijing, a number of websites appear to be making a veiled protest at state censorship by referring to the date sarcastically as "Chinese Internet Maintenance Day".

Earlier this week the government blocked access to a number of popular western websites, in what was widely seen as way of controlling access to information about the events at Tiananmen Square. Among the sites that were screened out were photo-sharing website Flickr, Microsoft's Hotmail email service and the popular online messaging site Twitter.

A number of other sites appear to have gone down over recent days, however, in a move that may be part of an ad hoc anniversary protest online.

According to Chinese media blog Danwei, the music-sharing service VeryCD has been taken down, as well as Fanfou, a local version of Twitter. But rather than simply fail to load, which would be typical for websites blocked by the firewall, many of the sites are now carrying messages saying that they are closed for maintenance.

It is not clear whether any of the sites took down their services as a result of government pressure: most have had previous trouble with the authorities in Beijing, and reports suggest that many sites were told that they would face serious consequences if they published anything relating to the events of 4 June 1989.

But it was also suggested that the phrasing used by some of the websites indicates a subtle attack on the government.

While deliberate government action cannot be ruled out, more than 300 Chinese sites appear to have posted increasingly blasé maintenance messages on the anniversary.

"The Fanfou server is undergoing technical maintenance. Service is expected to resume before dawn on 6 June," said one message. On dictionary website WordKu.com, its owners said they had taken the site down for Chinese Internet Maintenance Day.

Blog hosting service Bullog.org, meanwhile, says it has gone "on strike" for the day, and Wuqing.org carried a message saying: "I, too, am under maintenance!"

Internet users in China often deploy subtle methods to criticise the government without falling foul of the law.

Among the favoured techniques is repurposing internet slang to make fun of leading political figures or mock their policies.

In the past the term "grass mud horse" – a lewd pun intended as a jab at the censorship of bad language – gained currency among China's internet-savvy crowd. A string of empty government slogans, meanwhile, was parodied in a popular meme known as "the river crab wears three watches".